DESCRIPTION (adapted from the Abstract): The purpose of this cohort study is to explore epidemiologic, immunological, and virological characteristics of HIV-infected Thai women and to identify correlates of protective immunity which can contribute to intervention development. Approximately 125 HIV-1 infected pregnant women will be enrolled at the provincial hospital in Lampang, Thailand. The women will have blood samples, cervico-vaginal fluid, and colostrum collected during pregnancy and at delivery. Infants will be evaluated at birth and will be followed every two months for six months to classify their HIV status. Specific aims of the study are the determination of relative transmission risk associated with (1) elevated plasma viral burden, (2) absence of maternal antibody which neutralizes wild type viral isolates passaged in peripheral blood mononuclear cells, and (3) advanced maternal disease stage defined by CD4 count and clinical history. Subordinate aims to be evaluated in a preliminary fashion are: (1) to evaluate maternal behavioral risk factors for transmission to infants, 2) to compare total and class-specific antibody and fine mapped responses to HIV envelope among different mucosal compartments and serum and evaluate their relationship to transmission, and 3) to use envelope sequence analysis and determine which maternal compartment is most closely related to the infant HIV, in settings where peripartum transmission is likely.